Katie Couric was a smash hit Monday night sitting in for Leno as host of the “Tonight” show – more than 40 percent better than his usual Monday night ratings for the season to date.

In a much-publicized job swap, Leno flew to New York to co-host the “Today” show on Monday morning and Couric jetted to Los Angeles take over Leno’s late night gig for a night.

It was all part of a stunt cooked up by NBC brass to draw more viewers during the all-important May sweeps period.

But to everyone’s surprise, the stunt – especially on Couric’s end – proved a whopping audience draw.

Inevitably, her Nielsen performance sparked a tidal wave of buzz yesterday that a primetime show for the popular newswoman could not be far away.

It was the highest “Tonight” show since May 2000, the night of NBC’s season finales that year. “Tonight” hasn’t done better on a Monday night since 1996.

“It was an outstanding stunt- and she charmed the audience,” said former NBC entertainment chief Warren Littlefield. “It’s easy to see just how broad her talent range is,” Littlefield said. “The question for Katie is: What do you want to do now?”

It’s no secret that Couric has been flirting with leaving “Today” for some time. The grind of early-morning wake ups combined with a new, third-hour for the “Today” show have been weighing on her for some time.

“I think Katie Couric could be a success in whatever she undertakes,” said media analyst Tom DeCabia of ad firm Advanswers/PHD. “She’ll be able to actually have a career in another [part of the day],” he said.

In late 2001, shortly before Couric signed her record-setting, five-year, $65-million deal, she was also reportedly taking meetings with talk show producers who offered her a mint and a chance to be the next Oprah with her own syndicated daytime talk show.

The move would have easily turned Couric into one of the highest paid personalities on television.

Ultimately, insiders say, Couric’s decision stick with “Today” had less to do with money – although she did become the highest paid news personality on TV – than with staying at NBC and its well-established news organization.

Couric appeared relaxed and in control during her one-night stand as “Tonight” show host – the first guest host for the show since Leno took it over from Johnny Carson in 1992. She had no problem flashing her sex appeal, showing off her legs and even joking about her cleavage.

“Audiences respond to those things,” said Littlefield. “Katie is a superstar and it was a chance to see Katie in a role audiences hadn’t seen her in before.

“If she decided to do a movie of the week, there’s no question in my mind that there’d be a big audience for her as well.”

TALE OF THE TAPE

Katie Couric

Age: 46

Salary: $17 million a year

Early TV gig: Shunned as CNN correspondent and banned from the air for her squeaky voice

Hates being called: Perky

Car Fetish: New Thunderbird

Best “Tonight Show” joke: Points to her cleavage and says, “For all you people from L.A. who have never seen them before, these are actually real!”